The present invention relates to a printer in which a print head is provided to be pressed against or released from a print paper mounted on a platen to perform a print operation or an erase operation by moving the print head held in press contact with the print paper through a transfer medium such as an ink ribbon. This type of printer is disclosed in U.S. Pat. Nos. 4,384,797 and 4,396,308, for example.
Such a printer may include a thermal printer capable of performing the print operation by the flow of molten material from a transfer medium such as an ink ribbon to a print paper. Local heating of the transfer medium may be achieved by heating the print head or the transfer medium itself due to an electrical resistance against an electric current to be applied to the medium. A typical prior art thermal printer includes a carriage mounting thereon a thermal head (hereinafter referred to as "head" for short) and a ribbon cassette including a thermal ribbon (hereinafter referred to as "ribbon" for short) is wound on a supply spool and a take-up spool. The ribbon cassette is mounted on the carriage in such an arrangement that a portion of the ribbon exposed from the ribbon cassette may be located between the head and the platen. The carriage is movable along the platen.
In order to effect the print operation, the head is pressed against the platen through the ribbon and the print paper, and during the movement of the carriage held in this condition, the ribbon is heated to cause ink of the ribbon to melt and adhere to the print paper. Then, the ribbon is separated from the print paper to transfer the molten ink to the print paper. During this print operation, the ribbon is unwound from the supply spool and travels in a nipped condition between the head and the print paper to be wound onto the take-up spool.
In recent years, a thermal printer with an erase capability has been gradually put in practical use. In an example of this type printer, a single ribbon is used to effect both the print operation and the erase operation. In many cases, such a printer employs a ribbon having ink so formulated as to melt at a predetermined temperature and generate an adhesive force at an intermediate temperature between the print temperature and the room temperature. In the print operation, the ribbon is heated to the print temperature to transfer the ink of the ribbon onto the print paper, while in the erase operation, a character equal to that having been already printed or a solid mark as covering the periphery of such a printed character is reprinted on the printed character. When the temperature of the ribbon reaches the intermediate temperature, the ribbon is separated from the print paper, so that the adhesive force is generated in the ink on the print paper at this intermediate temperature to cause the ink of the printed character to adhere to the ribbon and be stripped off from the print paper.
The ribbon may be cooled to the intermediate temperature by various means such as by providing a correction blade for holding the ribbon having been heated by the head in contact with the print paper, while the head moves a predetermined distance, so that the ribbon may be cooled to the intermediate temperature during the movement of the head. Such a correction blade is disclosed in Japanese Utility Model Laid-Open Publication No. 62-114753, and the corresponding U.S. Application was filed on Dec. 29, 1986 (Ser. No. 947,915). This copending commonly owned application is a part of the disclosure of this application. There is provided another means wherein only during the erase operation, a moving speed of the carriage is reduced so as to prolong the period of time between heating of the ribbon and separating of the ribbon from the print paper and thereby provide an extra time for the cooling. In any cases as mentioned above, the adhesion is developed between the ribbon and the printed character, that is, there is generated a force acting to prevent ready separation of the ribbon from the print paper. As described above, a relatively strong adhesion is developed in the erase operation, and even in the print operation, melting and transfer of the ink causes adhesion between the ribbon and the print paper.
Conventionally, such a printer successively effects the print operation or the erase operation with no waiting steps interposed. Especially in a ribbon having improved print qualities for a rough print paper, the adhesive force of the ink is so high as to generate a close contact force to be developed between the ribbon and the print paper, and if the take-up force for the ribbon is not sufficient, the ribbon may stay in contact with the paper to be prevented from separation therefrom during the successive print or erase operation, which will result in defective winding of the ribbon.
Furthermore, as the ribbon moves in a nipped condition between the head and the print paper, any bias of the pressure distribution in the contact area between the head and the print paper will direct the force applied to the ribbon toward a portion under a lower pressure along the width of the ribbon. Consequently, during the successive print operation using a long ribbon, the ribbon will be gradually deviated from the normal travelling path to cause meandering, and in a serious case, it runs off the head, resulting in incapability of any further print operation.
Also, in the case that the used area of the ribbon along the width thereof is partial such as when printing a continuous underline, that is, in the case that the load generated between the ribbon and the print paper when the ribbon is separated from the paper is unbalanced along the width of the ribbon, such meandering will similarly occur.
In the erase operation, a relatively strong adhesion is developed to effect erasing as described above, resulting in a greater liability of defective winding and meandering.